1. Field of the Invention
This invention relates to a radio receiver apparatus and more particularly to a method for phase correction thereof.
2. Description of the Related Art
Conventionally, in a digital cellular system, which is one type of radio telephone, a voice signal is transmitted and received in a coded manner so that a channel can be used simultaneously by a plurality of terminal devices by applying the technique of time-division multiplexing. When power is applied, a terminal device of this type sequentially scans 124 channels previously set to detect a channel at which the electric field strength is most intense. The terminal device then detects a BCCH (broadcast control channel) assigned to an area to which the terminal device belongs and receives such BCCH.
The BCCH has time slots through which various information is transmitted. In the digital cellular system each terminal device receives information from the base station which transmits the BCCH, information on neighboring base stations and access information of terminal devices.
Thus, the terminal device detects an FCCH (frequency correction channel) which is included within the BCCH at a predetermined timing, to thereby generally detect a timing at which the necessary information is transmitted on the basis of the FCCH.
The FCCH is a synchronizing signal to which a bit pattern is assigned so that data of the value "1" continues for a predetermined number of bits when demodulated. The terminal device detects the timing of the FCCH to synchronize, i.e., frame synchronize, its overall operation with the timing of received data S.
Upon attaining frame synchronization by the FCCH in this manner, the terminal device synchronizes its overall operation with bursts.
A burst is output onto the BCCH at a predetermined timing and, as FIG. 1 shows, a training sequence is put in at substantially the center thereof. The training sequence is formed by repeating data of value "1" and value "0" in a previously set bit pattern, whereas the FCCH is formed by assigning a bit pattern so that data of value "1" continues for a predetermined number of bits.
This type of digital cellular system generally synchronizes its overall operation with the received data on the basis of the FCCH, and synchronizes the overall operation according to the bits received from the training sequence.
In particular, by detecting a correlation between a predetermined bit pattern and received data, the terminal device detects a timing at which the correlation value rises so as to detect the timing of a training sequence. Thereby, the terminal device detects the timing of the received data by bit, and synchronizes its overall operation with a burst by controlling and overall operation on the basis of the detected result of the correlation value.
The terminal device monitors the BCCH so that, upon detection of the access information from the base station, it is capable of communicating with a desired destination by shifting to a designated channel. Then, on the basis of a comparison with the reception of the BCCH of a neighboring base station, the terminal device switches to the BCCH offering the best reception, so that, even in case of moving into a neighboring area, the communication can be secured.
As FIG. 2 shows, data received at this type of terminal device is characterized by continuous fluctuation in its frequency due to fading.
Such fluctuation in frequency tends to be large in comparison with a burst and a frequency changed only by a certain value is maintained within one burst.
For this reason, in the case where data is processed at the terminal device by detecting the timing of a training sequence, even though data can be processed at a timing synchronized in phase with the received data within the training sequence, a phase shift between the timing of data processing and the timing of the received data may occur at both ends of a burst which are distant from the training sequence.
In this case, the bit error rate increases at both ends of the burst, thereby making it difficult to receive correct data.
Further, since this type of terminal device is formed as a superheterodyne system, a similar state occurs also in the case where a frequency offset of the local oscillating signal occurs.
In this case, for example, a method can be considered of correcting this type of phase shift by generating a clock as a reference for received data processing in a PLL circuit formed as an analog circuit. In this case, however, there is a problem that the overall construction becomes intricate and complicated, and the reliability thereof is also reduced since noise is increased and it is impossible to completely prevent deterioration due to aging of the elements forming the circuit.
On the other hand, another method can be also considered of estimating the amount of change in the frequency by sequentially detecting the amount of the phase shift so as to switch the timing of data processing by the estimated result. In this case, however, there is a disadvantage that operational processing becomes intricate and there can be a case of erroneously estimating the frequency, thereby causing a serious communication failure.
Furthermore, in this case where the amount of the phase shift is actually detected, there is also a disadvantage that the phase shift, with respect to the burst itself, which is utilized in the detection cannot be corrected by the detection results.